Field of the Invention
The present invention relates to a density correction technique in an image forming apparatus.
Description of the Related Art
In electrophotographic image forming apparatuses, misregistration correction and density correction are performed in order to maintain the quality of images to be formed. For example, density correction is performed by forming a density correction test pattern on an image carrier such as a photosensitive member or an intermediate transfer member, and detecting the density of the test pattern with an optical sensor to generate a tone correction table. Here, even if light is not incident on a light receiving element of the optical sensor, a minute current, that is, dark current, flows therethrough. This dark current fluctuates according to temperature, and therefore the voltage output by the optical sensor also changes due to changes in the temperature of the optical sensor. For this reason, Japanese Patent Laid-Open No. 2011-112988 discloses a configuration for measuring an offset voltage that occurs due to the dark current of the light receiving element prior to the detection of the test pattern, and detecting the density of a test pattern with consideration to the measured offset voltage.
In order to reduce the time taken for density correction by the image forming apparatus, there is a need to reduce the frequency of measuring the offset voltage that occurs due to the dark current of the light receiving element. For this reason, a configuration can be employed in which measurement of the offset voltage is carried out only when the image forming apparatus is powered on. Then, in the density correction, the density of the test pattern is obtained with reference to the offset voltage acquired at the time of power on. However, if the temperatures of the optical sensor at the time of power on and at the time of density correction are different, the value of the offset voltage that occurs due to the dark current also changes, and thus the error in the detected density of the test pattern increases. On the other hand, if the offset voltage that occurs due to the dark current is measured at each instance of density correction, the time taken for density correction increases and the downtime of the apparatus increases.